The invention relates to a method for detecting ATP and to a kit for use in this method.
A known method for detecting ATP (adenosine triphosphate) utilizes bioluminescence. The bioluminescence of luciferase/luciferin organisms is known, wherein luciferin is converted into oxyluciferin. This conversion is catalyzed by the enzyme luciferase.
A known luciferase originates from the firefly, Photinus pyralis . This luciferase is an enzyme having a molecular weight of about 60,000 dalton and containing about 550 amino acids. The substrate for this enzyme is luciferin, also originating from the firefly. This is a polyheterocyclic organic acid, D-(xe2x88x92)-2-(6xe2x80x2-hydroxy-2xe2x80x2-benzothiazolyl)-xcex942-thiazoline-4-carboxylic acid.
It is assumed that the luciferase-catalyzed bioluminescence-reaction proceeds as follows: 
Here, PP stands for pyrophosphate and AMP stands for adenosine monophosphate. The oxyluciferin is initially produced in an excited state. When the substance returns into its ground state, this is accompanied by the emission of light.
When ATP is present in a medium, the amount of ATP can be determined by means of the luciferin-luciferase reaction. When ATP or an ATP-containing medium is added to a composition containing luciferin, luciferase, magnesium ions and oxygen, a brief intense light signal is obtained, depending on the selected composition of the reagents. For a few seconds, one may speak of a relatively stable light signal. It is assumed that the decrease in the rate of the reaction is caused by a product inhibition.
When, by means of an injection system, an ATP-containing sample is added to the composition containing the reagents for the luciferin-luciferase reaction, the concentration of ATP in the sample can be determined. The drawback of the measuring system is that when a microtiter plate is used, each well must be measured separately, i.e. well by well. This is a particularly time-consuming business. When an ATP-containing sample is added to a number of reaction vessels or wells of microtiter plates, followed by measuring, the light signal of the last vessels or wells that are measured will already have disappeared entirely or largely.
Therefore, it is preferred that a relatively stable light signal be obtained in the luciferin-luciferase reaction. When the light signal is stable for a longer time, use can be made of plate luminometers which do not require an injection system. Moreover, one may then prepare a large number of reaction vessels or microtiter plates, to measure the light signal only some time later. The light signal then remains normative for the amount of ATP in the various samples.
The object of the invention is to provide a method for determining the concentration of ATP by means of the luciferin-luciferase reaction, wherein the duration of the light emission during the reaction is prolonged significantly.
In the literature, many attempts to influence and prolong the duration of the light signal in the luciferase reaction have already been described.
One approach which influences the kinetics of the luciferin-luciferase reaction and prolongs the time of the light emission utilizes an inhibitor for the luciferase enzyme. An example of an inhibitor which has received much attention is arsenate. Arsenate decreases the intensity of the flash and also prolongs the length thereof. However, the sensitivity of the reaction for detecting ATP decreases as well. Also, the use of arsenate is less desirable from an environmental viewpoint. Moreover, the decrease of the intensity of the light signal is considered undesirable, in particular when microtiter plates or instruments capable of reading out strips are used.
The kinetics of the luciferin-luciferase reaction have been studied by DeLuca et al. in Analytical Chemistry, 95, 194-198, 1979. The effects of the addition of arsenate and other ions are demonstrated by the authors. However, a drawback of the method proposed is the non-linear relation of the decrease of the light intensity over a wide concentration range of ATP determination.
U.S. Pat. No. 5,618,682 describes a method for detecting luciferase, wherein a reaction mixture containing ATP is used. The reaction mixture contains, in addition to ATP, adenosine monophosphate, a radical catcher (DTT), dithiothreitol and a chelating agent (EDTA). A protease inhibitor, such as phenylacetic acid, is used as well.
Another approach opts for the use of co-enzyme A (CoA). In Biochimica et Biophysica Acta, vol. 27 (1958), 519-532, Airth et al. have described that the addition of CoA to a luciferin-luciferase reaction mixture has no effect on the light flash, but that the intensity of the light signal after the flash remains at a higher level for a longer time.
This approach is also opted for in U.S. Pat. No. 5,650,289, which proposes techniques and compositions of reaction mixtures which influence the kinetics of the luciferin-luciferase reaction. The techniques described are based on the use of CoA and DTT. The half-life of the light signal, i.e. the time after which 50% of the original light signal is observed, is assessed at 300 to 500 seconds.
U.S. Pat. No. 4,246,340 likewise proposes a method for obtaining a half-life of the light signal in a luciferin-luciferase reaction of a few minutes. The method described is based on the use of inhibitors, such as D-luciferin analoga. An example hereof is L-luciferin.
As mentioned, the object of the present invention is to provide an improved method for detecting ATP by means of the luciferin-luciferase reaction, while a light signal is obtained which lasts significantly longer than is described in the prior art. In addition, it is an object of the invention to provide that the intended method is simple and can be performed in a well of a microtiter plate.
The invention is related to a method for detecting ATP in a sample, wherein the sample is contacted with a reaction mixture to effect a light signal, the reaction mixture comprising luciferin, luciferase and one or more water-soluble salts, the total salt concentration being at least 0.05 mole/liter, and wherein the light signal is measured.